1. Field of the Invention
The present invention relates generally to a Multimedia Broadcast/Multicast Service (MBMS). In particular, the present invention relates to a method in which a radio network controller (RNC) transmits MBMS data by considering a transmission time for each of a primary cell and neighbor cells in order to provide an MBMS service.
2. Description of the Related Art
Today, in accordance with developments in the communication industry, a service provided in a Code Division Multiple Access (CDMA) mobile communication system is evolving from a voice service into a multicasting multimedia communication service that is capable of transmitting large-volume data, such as packet data and circuit data. To support the multicasting multimedia communication service, a Broadcast/Multicast Service in which a service is provided from one data source to a plurality of user equipment (UE) has been proposed. Such Broadcast/Multicast Service can be divided into a Cell Broadcast Service (CBS) which is a message-oriented service, and a Multimedia Broadcast/Multicast Service (MBMS) supporting multimedia data such as real-time image and voice, still image, text, and the like.
The MBMS service refers to a service for transmitting the same multimedia data to a plurality of UEs through a wireless network. The MBMS service allows the UEs to share one radio channel, thereby saving radio resources. The CBS service refers to a service for broadcasting a plurality of messages to all UEs located in a particular service area. The particular service area providing the CBS service may include the entire area where the CBS service is provided in one cell. The MBMS service, which is a service for providing both voice data and image data, requires a large quantity of transmission resources. The MBMS service is provided through a broadcasting channel (BCH), given that a large number of services can be simultaneously developed within one cell.
FIG. 1 is a diagram illustrating a timing relationship in a process of transmitting packet data in a synchronous mobile communication system.
Generally, a 3rd generation (3G) mobile communication system uses a CDMA scheme. Standardization on the 3G CDMA mobile communication system is being separately carried out by the 3rd Generation Partnership Project (3GPP) which is a European standardization organization, and the 3rd Generation Partnership Project 2 (3GPP2) which is a US standard organization. The 3GPP system is called a Universal Mobile Telecommunication System (UMTS), in which, communication between the B Nodes is performed on an asynchronous basis. However, the 3GPP2 system is called a CDMA2000 system, in which, communication between base stations (or B Nodes) is achieved on a synchronous basis. A distinction between the synchronous system and the asynchronous system is made according to whether external synchronization time information is received from a Global Positioning Satellite (GPS) system to maintain synchronization.
In the synchronization system, all base stations use the same pseudo-random noise (PN) code, but each base station uses a PN code with a unique phase offset through phase delay. However, in the asynchronous system, all B Nodes (or base stations) use different PN codes for Node B identification.
FIG. 2 is a diagram illustrating cells performing soft combining in a general mobile communication system. In FIG. 2, a particular UE is located in a cell 201, and the cell 201 supports an MBMS service requested by the UE. The neighbor cells 202 and 203 of the cell 201 also provide the MBMS service requested by the UE. The cells 201, 202 and 203 can be defined as an active set of the UE. An RNC controlling a plurality of cells can define the cells 201, 202 and 203 as a soft combining region for reliability of MBMS data for the UE.
When the soft combining region (or group) A (that is, the region including the cells 201, 202 and 203) performs soft combining for an MBMS service, not only does the cell 201 of the UE transmit the same MBMS data frame for the same time, but also the neighbor cells 202 and 203. This is because each cell transmits an MBMS data stream to the UE located in the soft combining region according to external reference time information due to a characteristic of the synchronous system.
Referring to FIGS. 1 and 2, if the UE transmits an MBMS service request and then detects transmission of data for the MBMS service, an RNC receives MBMS data from a core network which comprises its upper system. The received MBMS data is delivered to a Node B (herein, being equal to a cell in terms of the concept) that manages a cell in which a UE that requested the MBMS service is located. Therefore, a Node B1 101 receives an MBMS data stream transmitted from the RNC which comprises its upper system, and stores the MBMS data stream in a buffer therein. It is assumed herein that a size of the received MBMS data stream is P1+P2+P3, and that the buffer operates on a first-in first-out (FIFO) basis. A Node B2 102 also stores an MBMS data stream transmitted from the RNC in a buffer therein. A size of the MBMS data stream received at the Node B2 102 is P1+P2. Further, a Node B3 103 also stores an MBMS data stream received from the RNC in a buffer therein.
If a service notification for the MBMS service is received, each Node B starts transmitting the MBMS data to the UE at a service start time T0. The Node B1 101 transmits the MBMS data at a start time T0=T1. The Node B2 102 transmits the MBMS data at a start time T0=T2. The Node B3 103 transmits the MBMS data at a start time T0=T3. That is, in the exemplary synchronous mobile communication system described above, MBMS data transmission start times of the B Nodes 101, 102 and 103 are fixed to T0=T1, T0=T2 and T0=T3, respectively, based on the external reference time information.
The UE receiving MBMS data from the cells performs soft combining on the received MBMS data, thereby guaranteeing reliability of the received data.
An asynchronous mobile communication system supporting an MBMS service requires the soft combining scheme for guaranteeing reliability of MBMS data for a UE.
However, the asynchronous mobile communication system supporting an MBMS service cannot acquire fixed transmission times using external reference time information. That is, as described above, because the asynchronous mobile communication system basically does not use external standard time information from the GPS system, each Node B thereof transmits a frame according to internal time information. Therefore, the asynchronous mobile communication system cannot support an MBMS soft combining scheme for transmitting MBMS data based on external reference time information.
Accordingly, a need exists for a system and method for transmitting the MBMS data at the same transmission time so that the UE can receive requested MBMS data.